The invention relates to a device for fixing a toner image on a carrier web, comprising a fixing chamber in which toner images present on the carrier web are charged with solvent vapor, the carrier web being supplied to and carried away from the fixing chamber via a respective gap. Further, the invention relates to a method for fixing toner images.
Aside from the fixing of toner images by a contacting heat-pressure treatment and a contactless heat treatment, contactless cold fixing has also gained importance. During this cold fixing, the toner material is softened under the influence of a solvent. The softened toner moistens the surface of the carrier material. When using a carrier material that contains fibers such as, for example, paper or textiles, the softened toner surrounds the fibers and, due to capillary forces, partly penetrates into the spaces between the fibers and into them. After the drying of the carrier material and the evaporation of the solvent, the toner again congeals and solidifies. In this way, the toner is joined to the carrier material in a smear-proof and abrasion-resistant manner.
The presence of the solvent in vapor form is advantageous since a condensation of the solvent vapor onto the toner particles takes place so that the vapor molecules directly deposit on these toner particles. Moreover, the output of the evaporation enthalpy in the condensation supports the softening of the toner and increases the speed of the dissolving process. A further advantage of fixing with the assistance of a solvent vapor is the little thermal stress on the carrier material, from where also the name cold fixing comes from. Accordingly, carrier materials can be employed that withstand only a little thermal or mechanical load such as, for example, labels or films. Moreover, the moisture content of the carrier material is not changed so that a waviness arising due to changes in moisture is avoided. Also, cold fixing is largely independent of the thickness of the carrier material so that, for example, papers having different paper thicknesses can be used without a great modification of the fixing process. In this way, a change in the type of paper can also take place with little expense.
WO 02/10862 A1 of the same applicant discloses a device and a method for fixing a toner image on a carrier material by using solvent vapor. A directed stream containing the solvent vapor is generated, the stream being directed to a section of the carrier material with the aid of a nozzle device. The device described and the method are suited both for simplex printing as well as for duplex printing. The patent application mentioned is herewith incorporated by reference into the disclosure of the present application.
Due to the vapor generation, a slight overpressure is generated in the inside of the fixing chamber, this overpressure having the consequence that the solvent vapor exits the fixing chamber, in particular at the relatively large gaps for the entrance and the exit of the carrier web. These gaps are inevitably present. In the case when the carrier web is printed on both sides and is linearly guided through the fixing chamber, the gap width cannot be designed too narrow. At the surface of the entering carrier web, i.e. at the upper and lower boundary layer, ambient air is captured from outside and brought into the fixing chamber at the entrance gap by means of viscous friction. Due to the constant volume in the fixing chamber, this has the consequence that a corresponding volume, which is enriched with solvent vapor, is forced outwardly from the inside of the fixing chamber at the exit gap. Furthermore, solvent that has reached into the boundary layer at the carrier web in the fixing chamber is dragged out from the fixing chamber at the exit gap together with the carrier web.
For environmental reasons as well as for economic reasons, the escape of solvent from the fixing chamber into the environment, so-called leakages, have to be largely avoided. This means that the leakage streams have to be reduced or the solvent concentration of leakage streams that cannot be avoided has to be kept at a value as low as possible. This requirement is particularly difficult to meet at the entrance gap and the exit gap for the carrier web. A contacting or dragging seal, for example by means of a sealing lip or a sealing roller, is not possible at these points. At the entrance gap, the powder toner merely lies loosely on the carrier web and would be smeared by a dragging seal. A contacting, synchronously running sealing roller by which no toner is removed can only be realized at a high economic expense. At the exit gap, the toner is indeed smear-proof to a certain extent after fixing, but still relatively soft. A dragging sealing lip might result in a toner removal. As shown by experiments, a contacting, rotating sealing roller is quickly covered with toner material if, as is common, it is charged with a high solvent concentration on the side facing the fixing chamber.
The amount of solvent dragged out at the carrier web entrance and at the carrier web exit has to be compensated for in order to maintain the high solvent concentration required for the fixing inside the fixing chamber. In general, this leads to a considerable solvent consumption which in turn causes high operating expenses. In addition, the dragged-out solvent can have a negative impact on the ambient air. In order to be able to keep the admissible work place concentrations for solvents, the fresh air content in the room air has to be very high, which again causes high operating expenses.
FIGS. 1 and 2 show known examples of the prior art. FIG. 1 schematically illustrates the principle of cold fixing. A carrier web 10, in general a paper web, is provided with toner images 14 on both sides in a transfer printing station 12 indicated by a roller pair. Via an entrance gap 16, the carrier web 10 enters a fixing chamber 18, in which a solvent vapor having a high solvent concentration is present.
In the fixing chamber 18 suitable solvents, such as isopropanol, acetone or ethylacetate, are evaporated and the solvent vapor is directed to the upper side and the lower side of the carrier web 10. There, the powdered toner present on the surface is softened by the solvent vapor and is bound on the carrier web. Further details with regard thereto can be taken from WO 02/10862 A1 of the same applicant, the content of which is herewith incorporated by reference into the disclosure of the present application.
The carrier web 10 exits the fixing chamber 18 via an exit gap 20. A roller pair 22 guides the carrier web 10 and generates the web tension required for the contactless fixing. As a result of the vapor generation, a slight overpressure arises in the fixing chamber 18, this overpressure having the effect that solvent is dragged out via the entrance gap 16 and the exit gap 20.
FIG. 2 shows an example according to the prior art as described in U.S. Pat. No. 4,311,723 of the same applicant. This document, too, is incorporated by reference into the disclosure of the present application. In the following, same parts have the same reference signs.
The fixing chamber 18 is designed as a vertically oriented container in which solvent is conducted into the inside of the fixing chamber 18 with the aid of a pump 24. The solvent is evaporated on the bottom which is designed as a heating plate 26. Cooling tubes 28 for cooling the solvent vapor are arranged within the fixing chamber 18. Accordingly, the solvent concentration in the upper region 30 of the fixing chamber is lower than in the middle region 32 and lower than in the lower region 34. Thus, the highest solvent concentration is present in this bottom region 34. The carrier web 10 horizontally enters the fixing chamber 18 and is deflected vertically downwardly at a first deflection unit 36 and is guided into the area with a high solvent concentration in the bottom region 34. The toner images are partly fixed during this path of the carrier web 10. The carrier web 10 is again deflected at a second deflection unit 38 and is finally led out of the fixing chamber 18 again via a third deflection unit 40.
By means of the vertical web guidance, the exit of solvent along the arrows P1, P2 is reduced since the solvent concentration is the highest in the bottom region 34. This reduction in solvent concentration in the upper region is assisted by the cooling tubes 28 which form a cold trap in the upper region 30 of the fixing chamber 18. Thus, the solvent concentration in the region of the entrance and the exit of the carrier web 10 is further reduced. With this arrangement, however, the boundary layer at the surface of the carrier web 10 is not reached by the cooling so that the solvent concentration is not reduced thereat, which results in a considerable solvent drag-out in the case of a relatively high transport speed of the carrier web 10. Further, with this arrangement, a deflection-free, horizontal web guidance as would be required for toner images present on both sides of the carrier web 10 is not possible.
The previous solutions for the problems described are deficient. The entrance gap and the exit gap cannot be designed arbitrarily narrow, since the passing carrier web, such as paper, presents some forms of unevenness such as waviness over the paper width and can swing or flutter. In case of a contact with the gap edges, an abrasion or a smearing of the toner on the carrier web can be caused.